My timing curve before any adjustment.....

[sc2dave]

I got these numbers BEFORE I noticed that the setting on the Sun Super Tach was at the 4 cyl setting. It was too late in the evening to adjust it to 6 cyl. At 1,000 I had 41*, 1500 was 41, 20000 was 46,2500 was 48 with vac adv attached to manifold vacuum. Without vac adv, I had 12, at 10000,12 at 1500 I had 14, 2000 at 19* 2500 was like 20*,30000 was 23*. How messed up is this? 194 cid ,rv cam,headers,Offy intake adjustable vac can.

[rebelrouser]

First I never check the advance curve with the vacuum advance hooked up. Under hard acceleration you should not have any vacuum. Vacuum advance is for part throttle fuel economy. I want to rev the engine until the mechanical advance is all done, set the max timing to where you want it, 32 to 34 degrees is common. Let it come down to idle and see what the idle timing is. Then rev the engine in 500rpm or so and plot your curve. The right curve is trial and error sometimes because a lot of things affect it. I have had good luck using performance trends engine program, as it calculates an advance curve for the engine. You can change octane and fuel types, or put in boost etc. I have found it in the ball park on most engines I have done.

http://www.enginebuildermag.com/2017/09 ... g-science/

[sc2dave]

I switched the tach to 6 cyl and redid the numbers again. W/O vac adv, 12* initial timing. At 1000 rpm 14, at 1500 18*, at 22,at 2500 25*,at 3000 26*. With vac adv, idle was 40*, 1000 was 43, 1500 was 48, 2000 was 52, 2500 was 55*, 3000 was 56*. Are these numbers too much? I never get pinging.

[groberts101]

I switched the tach to 6 cyl and redid the numbers again. W/O vac adv, 12* initial timing. At 1000 rpm 14, at 1500 18*, at 22,at 2500 25*,at 3000 26*. With vac adv, idle was 40*, 1000 was 43, 1500 was 48, 2000 was 52, 2500 was 55*, 3000 was 56*. Are these numbers too much? I never get pinging.

Of course the motor will never ping when you get deeper into the throttle because all that vac advance supplementation falls away as manifold vac drops. Unfortunately that also means you're leaving power on the table with such low base timing and lazy mechanical sweep.

Jack the base timing up.. quicken and shorten the curve to hit the total number you want.. and then SUPPLEMENT light throttle advance with a customized or adjustable aftermarket vac pot which gives around 10-15 more degrees of assist. Motor will pick up everywhere, especially during heavier throttle.